U.S. and Western weapons have been reaching Iranian-backed Shiite militias fighting to keep Bashar Assad’s forces in power in Syria.

Analysts say it’s unclear if the weapons were captured, stolen or bought on the black market in Syria, Turkey, Iraq or Libya. Propaganda photographs from Shiite militias posted on dozens of websites and Facebook pages show the weapons were acquired in new condition, said Phillip Smyth, an analyst for Jihadology.net, a site affiliated with the Washington Institute for Near East Policy.

Many of the weapons are things the militias “shouldn’t really have their hands on,” Smyth said. Iranians love to show “they have weapons and systems that are very close to the Americans.”

The ability of Assad’s allies to obtain U.S. weapons is one of many reasons the United States should not supply Syrian rebels with weapons, which President Obama said he would start to do last month, said Rep. Ileana Ros-Lehtinen, R-Fla., former chairwoman of the House Foreign Affairs Committee.

Syria is “already overflowing” with weapons being supplied to the Assad regime and to the rebels “that could one day be turned against the U.S.,” Ros-Lehtinen said.

It’s “extremely difficult” to distinguish between friend and foe in Syria, she said, and “no amount of safeguards can guarantee that weapons will not fall into the wrong hands.”

State Department spokeswoman Jen Psaki said the administration has taken steps “to do everything possible to ensure that any aid is making its way into the right hands” in Syria. That is why the United States and its partners have agreed to direct military aid through the secular-leaning, anti-Assad Free Syrian Army’s Supreme Military Council and its chief, Gen. Salim Idris.

Many of the U.S. weapons in the hands of pro-Assad militia could have reached the black market after a major U.S. sales to Iraq in 2009, said Christopher Harmer, a senior analyst at the Institute for the Study of War. Almost 10 years of fighting there left thousands of loose weapons floating around Iraq and available for sale on the black market.